The present invention relates generally to devices for neutralizing static electrical charge on a surface. In a particular aspect the present invention is directed to a device for removing or reducing static electrical charge buildup on a piece of paper in an office machine. In a further aspect, the present invention is directed to a machine including at least one electrical component susceptible to being electrically shorted by contact with conductive fibrous material and at least one device for neutralizing static electrical charge on a surface within the machine.
Static neutralizing or discharge devices have been used to remove any static charge or buildup that may occur on copy sheets, or other paper sheets that may be used in the office machine. Since paper is a dielectric material, static electrical charges are generated by contact with various parts of the office machine. This typically happens by frictional contact with the guide members and transport devices, for example. In addition, charge may be created on a paper sheet as a result of operations performed on it. For example, in electrostatographic reproducing machines the copy sheets are subjected to numerous electrostatic charge and discharge operations which can result in creating charge on a paper. The static charge generated at each process step may be either positive or negative and since the paper is thin and flexible it will be repelled from some objects or surfaces and attracted to other objects and surfaces resulting in unpredictable sheet handling. This propensity of the dielectric paper to accept triboelectric charge and hold it presents two problems to the user. The buildup of electrical charges on the sheets can cause the sheets to tend to stick together thereby increasing the difficulty in physically handling the sheets from station to station in an office machine. In addition, with the number of charged sheets being grouped together as in a collector tray, a spark may flow to a human operator, for example, who may come in contact with the sheets. This presents an undesirable shock hazzard to the operator. Furthermore the static electrical charges on the copy sheets attract dust and other machine debris which can end up on the sheet produced from the machine. For example, in an automatic reproducing apparatus using the electrostatographic process, dirt and other machine debris present on the copy sheet prior to the transfer of fusing of the copy of the sheet may result in substantial contamination of the sheet by the dirt and debris. Furthermore with the buildup of charge on copy sheets in an automatic reproducing machine, toner marking material may end up in areas of the sheet where you do not desire it, causing undesirable background and thereby reducing the copy quality. In addition, of course, with successive sheets of paper being electrostatically held together by static electrical charges, the ease with which the copy paper may be handled is dramatically reduced.
It has been previously proposed to eliminate or substantially reduce the propensity for operator shock, to increase paper handling ease, and to also increase copy quality through the use of several devices. Initially these devices took the form of active powered devices which ionize the air around the surface of the copy paper to afford a path to ground through the ionized air. The principal disadvantages of these devices is that they require a separate power supply, produce ozone, and are relatively expensive to include in the machine and to operate. It has also been common practice to employ a "tinsel" type of device at the output end of the machine predominately to dissipate charge in advance of the operator coming into contact or proximity to the copy sheet. These tinsel devices typically take the form of a plurality of metal projections on an electrically grounded support which are positioned transverse to the path of the sheet or web and physically contact the sheet. Performance wise of course, this leaves much to be desired since the electrostatic charge on the copy paper has not been removed until the copy paper has left the automatic machine. In addition, the physical contact may scratch or otherwise deface the sheet. During the processing of the image on the copy paper in an automatic reproducing machine, brute force to insure positive paper handling of the paper is required. Thus, paper handling in such machines requires extensive use of grippers, air puffers and other like devices to physically and very positively handle the copy sheet adding dramatically to the cost and complexity of such machines.
It has also been proposed to use brush type static eliminators in such devices and in particular brushes made of thin stainless steel fibers. These work very well in certain applications and provide sufficient neutralization of the static charge buildup on the copy sheet for many purposes. However, for high image machines producing thousands of copies on paper a noticable fall-off in the performance of the stainless steel eliminator brushes is observed after periods of continued use. In such situations after say 20,000 to 50,000 copies of paper have been processed, the stainless steel type static eliminator while capable of reducing the static charge buildup does not reduce it to a sufficient degree and again, difficulties with regard to the handling of paper occur. In automatic office machines, and in particular electrostatographic copying devices, improved reliablity in terms of physical handling of the copy document has always been sought. The presence of static charge on the paper sheets being handled, and particularly on successive paper sheets, has always been an impediment to achieving high reliability in such a machine.
In an attempt to improve on the static eliminator performance of the stainless steel fibrous brush, it has been suggested to use conductive carbon fibers. It was suggested that the conductive carbon fibers would have better wear life than their stainless steel counterparts, could be used for very long copy cycles such as, for example, the life of the machine. Such carbon fibers typically have steady state DC volume resistivities of the order of 10.sup.-2 to 10.sup.-3 ohm-cm. Static eliminator brushes made of such conductive fibers are capable of functioning to a certain degree, however, they suffer from the deficiencies in that the fibers are thin in diameter and brittle, and therefore the brushes tend to shed. In addition, these fibers typically have elongations less than 2 percent which contribute substantially to the brittle nature of the fiber and therefore brush shedding characteristics. This causes a problem in particular with regard to the charging devices in automatic reproducing machines in that if a shed conductive fiber comes in contact with the charging wire, it has a tendency to arc causing a hot spot on the wire resulting in melting of the wire and breaking of the corotron. Thus the fiber disintegrates, contaminates the device and disrupts uniformity of the corona charge. This is destructive irreversible damage requiring unscheduled service on the machine. This problem is dramatically encountered when the dicorotron type charging devices described in U.S. Pat. No. 4,086,650, the disclosure of which is totally incorporated herein by reference, are used in a machine having conductive fibers in a static eliminator. U.S. Pat. No. 4,086,650 described a dicorotron type charging device including a discharge electrode such as a conductive wire which has a relatively thick coating of dielectric material such as glass such that substantially no conduction current or DC charging current is permitted therethrough. The discharge device has a conductive shield adjacent the electrode and the imaging surface is charged by means of a displacement current or capacitive coupling through the dielectric material. With the static eliminators using brushes made from conductive carbon fibers, any fibers that shed or break off upon contacting the shield and wire, bridge the shield and wire or the ground and wire. The results in an instantaneous short producing an arc that produces an irreversible damage to the wire. Further the glass on the wire cracks and in a severe case the wire can be severed. The problem is compounded by the fact that the small fibers are difficult to percieve by the unaided eye and the degree of contamination by them cannot be discovered. The difficulty in electrostatographic reproducing machines is also compounded in that the several operations performed on the copy paper as it moves through the machine, provides ample opportunity to distribute the shed fibers throughout the machine increasing the possibility of electrical component failure.